Uterine contractility is affected by a variety of hormones. Substances such as beta-adrenergic agents which promote relaxation also increase uterine cAMP levels. Agents such as prostaglandins and oxytocin, which cause contractions have been shown in certain instances to antagonize relaxant-generated cAMP levels and may also elevate cGMP levels. The pregnant rat uterus shows decreased spontaneous contractions and responsiveness to oxytocin relative to nonpregnant uterus, measured as antagonism of isoproterenol effects on contractility and cAMP levels. Recent evidence from biochemical and physiological experiments suggested that a rapidly acting substance acts in addition to progesterone to achieve this effect. Uterine relaxing factor (URF) and relaxin, both of ovarian origin, have been variously described as identical and distinguishable, homogenous and resolvable into closely related polypeptides. The objectives of this study are to explore the mechanism of action of relaxin as it relates to the role of cyclic nucleotides in the regulation of uterine motility in the rat and to study the biochemical interrelationships between hormones known to interact in the uterus.